Industry demand for higher black liquor processing capacity in Kraft recovery boilers has caused a significant increase in furnace shaft velocity and particle carryover. Demand for higher energy efficiency in recovery boilers has also increased black liquor solids content delivered to the boiler, causing smaller mass-median droplet size from black liquor spray nozzles and a further increase in carryover. Higher carryover results in an increase in fouling and plugging of convection pass surfaces.
For a general discussion of chemical and heat recovery in the pulp and paper industry, and the particular aspects of the alkaline pulping and chemical recovery process, reference is made to Steam/its Generation and Use, 41st Ed., Kitto and Stultz, Eds., Copyright© 2005, The Babcock & Wilcox Company, Chapter 28, the text of which is hereby incorporated by reference as though fully set forth herein. FIG. 1 is sectional side view of a known design of a Kraft recovery boiler manufactured by The Babcock & Wilcox Company, generally designated 1. The two main functions of a Kraft process recovery boiler, soda process recovery boiler, or simply, “recovery boiler”, are to burn the organic portion of black liquor (a by-product of chemical pulping) to release energy for generating steam and to reduce the oxidized inorganic portion of black liquor in a pile, or bed, supported by the furnace floor. The molten inorganic chemicals in the bed, known as smelt, are discharged to a tank of water where they are dissolved and recovered as green liquor.
The recovery boiler 1 illustrated in FIG. 1 comprises a furnace 10 which is typically rectangular in cross-section, having enclosure walls 12 formed of water or steam-cooled tubes. The black liquor is fed into a lower portion of the furnace 10 through one or more black liquor BL spray nozzles 14 (also referred to as “black liquor guns”, “liquor guns”, or simply BL nozzles 14) which spray the black liquor into the furnace 10 through openings in the enclosure walls 12. The furnace 10 is generally rectangular in cross-section, and has a front wall 16, a rear wall 18 and two side walls 20. The front of the recovery boiler 1 is defined as the left hand side of FIG. 1, the rear of the recovery boiler 1 is defined as the right hand side of FIG. 1, and the width of the recovery boiler 1 is perpendicular to the plane of the paper on which FIG. 1 is drawn. The left hand side wall LHSW of the boiler 1 is defined as that side wall 20 on the left as one faces the front of the recovery boiler 1, and the right hand side wall RHSW is defined as that side wall 20 on the right.
With a conventional air system, combustion air is introduced into the recovery boiler 1 furnace 10 via air ports at staged elevations above a floor 22 of the furnace 10. These elevations are—primary air 24, secondary air 26, and tertiary air 28, as shown in FIG. 1. Typically, one fourth to one half of the air enters at the primary air PA level 24 near the furnace floor 22. The balance of the air for combustion is staged at the secondary air SA 26 and tertiary air TA 28 levels. The last stage or elevation of air, the tertiary air TA 28, is typically introduced at an elevation seven to fourteen feet above the black liquor nozzles 14. Good penetration and mixing of the tertiary air TA is needed to complete combustion of the black liquor and combustible gases (CO, H2, and H2S) within the furnace 10. The gases generated by combustion rise out of the furnace 10 and flow across convection heat transfer surfaces. Superheater surface 30 is arranged at the entrance to the convection pass, followed by steam generating (boiler bank) surface 32 and finally economizer surface 34. The water-cooled furnace enclosure walls 12 and the volume of the furnace 10 provide the necessary surface and retention time to cool the gas to temperatures where sootblowers can effectively remove the chemical ash from the convection surfaces. A furnace arch or nose 36 shields the superheater surface 30 from the radiant heat of the furnace 10 and uniformly distributes the gas flow entering the superheater surface 30.
The BL nozzles 14 produce a spray with a distribution in droplet size and a mass median droplet size of about 2-4 mm. Large particles (e.g. >3 mm) from the BL nozzles have downward trajectories because they are mostly influenced by their initial momentum and by gravity. The smallest particles of black liquor (<1 mm) are mostly influenced by aerodynamic drag forces and are lifted upward with the gas flow. These particles are known as carryover. Carryover particles are deposited on convection pass surfaces, which cause fouling and plugging of those surfaces and is detrimental to boiler heat transfer performance and continuous operation of the boiler. An air system design influences the quantity of particle carryover in two ways: 1) the magnitude and distribution of vertical velocity that provides lift for small particles, and 2) horizontal gas currents that push small particles towards the furnace walls, where they are deposited and removed from the gas.
Conventional air systems with the described three levels of combustion air typically have just one level of tertiary air above the black liquor nozzles 14, the principal function of which is to provide air to complete combustion of combustible gases and particles which ascend in the furnace. Formetti et al., U.S. Pat. No. 5,715,763 discloses a black liquor recovery boiler furnace having quaternary air injection ports located in the furnace walls in the vicinity of, or at approximately the same elevation as, the black liquor injection guns. Blackwell et al., U.S. Pat. No. 5,121,700, discloses a method of introducing air into a recovery boiler furnace which introduces air via sets of small and large jets on opposite walls, the small jets opposing large jets, and which is referred to in the patent as partial interlacing. The concept of vertically aligned air ports was originated by E. Uppstu et al. 1995, and is exclusively applied to secondary air systems in United States Patent Application Publication US 2004/0149185, and U.S. Pat. No. 6,742,463.